This invention relates to an oil well pipe joint for use in connecting oil well casing and tubing which are forced deep into the ground in the production of natural gas as well as crude oil.
In recent years, the depths of oil wells have increased to upwards of several thousand meters, sometimes reaching 10,000 meters below the ground.
The number of oil well casing and tubing members required to make up one oil well is accordingly also increasing. A large number of these pipes are connected in series by screw-type pipe joints.
Due to the great depths and lengths of piping, such pipe joints are exposed to severe conditions in which tensile load is applied in the axial direction due to the combined weight of the casing or tubing and joints therefor. In addition, compressive load due to soil pressure, etc. is also applied to the peripheral surface of each joint. Also, expansion load due to the pressure of the fluid passing through the joint is applied to the inner surface of the joint. These forces increase as the depth of the oil well increases. Nowadays, it is not unusual to exploit crude oil and natural gas under corrosive conditions containing H.sub.2 S and CO.sub.2 at a high pressure. Thus, pipe joints for use in connecting oil well pipes have to resist such severe corrosive conditions, too.
Therefore, it is very important that even under these severe conditions pipe joints be able to resist high tensile stresses and resist corrosion so as to reliably seal against not only liquids but also gases.
In addition, since connecting and disconnecting are carried out repeatedly, pipe joints should also have satisfactory sealing properties after repeated connecting and disconnecting.
There have been proposed a variety of measures to meet these requirements. For example, Japanese Laid-Open Patent Application No. 56-109975 discloses, as shown in FIG. 1, an oil well pipe of the coupling type which comprises as part of the oil well pipe an edge portion 2 and a shoulder portion 3 on which soft metals or alloys 4, 5 are respectively deposited by means of a metallizing method. In addition, the angle ".theta." between the end surface of the edge portion 2 of the pipe 1 and a shoulder portion 9 of the coupling 6 is adjusted to be within 15 degrees. The connection of the pipe 1 with the coupling 6 is carried out by screwing an external thread 7 of the pipe 1 into an internal thread 8 of the coupling 6. In the screwed position, the edge portion 2 of the pipe 1 and the shoulder portion 9 of the coupling 6 cause the soft metal layer 4 to flow into a clearance provided between the edge portion and the shoulder portion, resulting in packing of the soft metal 4 within the clearance to provide sealing therebetween. Furthermore, between the shoulder 3 of the pipe 1 and the edge portion 10 of the coupling 6, the soft metal layer 5 flows into a clearance therebetween to fill it and establish sealing therein.
According to the disclosure of that invention, the sealing of pipe joints is ensured by the above structure. However, a lip portion which constitutes a metal sealing portion is not provided, and the occurrence of galling, which sometimes takes place upon connecting the pipe 1 with the coupling 6, is not taken into consideration. The soft metal layers 4, 5 easily wear out during connecting due to frictional heat, causing galling and a loss in sealing properties. Namely, the seal between the pipe 1 and the coupling 6 is mainly achieved by the contact of the edge portion 2 and shoulder 3 of the pipe 1, respectively, with the shoulder 9 and edge portion 10 of the coupling 6.
Thus, the seal between the external thread 7 and the internal thread 8 is not so reliable as expected. Once the seal between these edges and shoulders is lost due to galling during connecting or disconnecting, the reliability of the sealing properties of the pipes as a whole will be easily lost, sometimes resulting in oil or gas leakage. In addition, it is impossible to use the pipe 1 more than ten times, since there is no means provided to overcome the galling problem in this type of pipe joint.
In contrast, in the pipe joint disclosed in Japanese Laid-Open Utility Model Application No. 57-122884, as shown in FIG. 2, the sealing as well as the corrosion resistance thereof are ensured by the provision of a layer of a heat-, pressure-, and abrasion-resistant resin, such as a fluoroplastic, on threaded portions 13 and 13' either of pipes 11, 11', or coupling 12, or butt portions 14 of the pipes 11 and 11', the pipes 11, 11' and the coupling 12 constituting part of an oil well pipe. However, the pipe joint of this type does not have a lip portion, either, which constitutes a metal sealing portion, and the design does not take into consideration the occurrence of galling during connecting and disconnecting. Thus, when galling takes place, the resin layer wears away and the sealing properties will be lost easily.
U.S. Pat. No. 4,474,651 discloses an oil well pipe joint comprising a metal sealing portion provided with a metal plating layer. In that invention, the metal plating is applied not only to the metal sealing portion, but also to the threaded portions. Furthermore, both an externally-threaded member and an internally-threaded member are provided with the metal plating. That U.S. patent states that such structure is preferable.